What is Ohm's Law and where does it apply?

Ohm's Law states V = I × R. It applies to linear resistive components such as resistors, conductors and heating elements. For AC reactive circuits, impedance Z is used instead of pure resistance R.

How do I calculate resistance from voltage and power?

Use R = V² / P. For example, a 100 W load on 230 V has a resistance of 230² / 100 = 529 ohms.

What is the difference between voltage, current and resistance?

Voltage is electrical potential difference, current is the flow of charge, and resistance is opposition to that flow.

How do I calculate the correct resistor for an LED?

Use R = (supply voltage − LED forward voltage) / LED current. For 5 V supply, 2 V LED and 20 mA current, R = 150 ohms.

Does Ohm's Law work for AC circuits?

Yes for purely resistive AC loads. For inductors and capacitors, use impedance Z instead of resistance R.

Ohm's Law Calculator — Voltage, Current, Resistance & Power

Ohm's Law Calculator

Enter any 2 known values — Voltage, Current, Resistance, or Power — to instantly solve for all 4.

⚡ Key Insight: Ohm's Law states V = I × R. Combined with the power equation P = V × I, you can derive any of the 12 possible formulas for V, I, R, and P using only two known values. This calculator handles all 12 automatically.

🔵 Ohm's Law — Visual Circuit & Triangle

Formula Wheel

Voltage (V) Current (I) Power (P) Resistance (R)

Voltage V

Current I

Resistance R

Power P

Enter any 2 values to calculate.

For AC inductive or capacitive circuits, use impedance Z instead of resistance R. For pure resistive loads, R and Z are effectively the same.

⚠ Please enter at least 2 known values.

Voltage

—

volts (V)

Current

—

amperes (A)

Resistance

—

ohms (Ω)

Power

—

watts (W)

📐 All 12 Ohm's Law Formulas

Voltage (V)

V = I × R
V = P / I
V = √(P × R)

Current (I)

I = V / R
I = P / V
I = √(P / R)

Resistance (R)

R = V / I
R = V² / P
R = P / I²

Power (P)

P = V × I
P = I² × R
P = V² / R

📋 Quick Reference

⚡ Common Voltages

AA / AAA battery1.5 V

9V battery9 V

Car battery12 V

USB (standard)5 V

UK mains230 V

US mains120 V

3-Phase industrial400 V

🔴 Resistor Colour Codes

Black0

Brown1

Red2

Orange3

Yellow4

Green5

Blue / Violet / Grey6 / 7 / 8

🔋 Typical Component Currents

Standard LED10–30 mA

Logic IC (TTL)1–20 mA

Arduino pin max40 mA

Small DC motor0.1–1 A

Laptop adapter3–5 A

EV charger16–32 A

Industrial motor10–500 A

🟢 Wire Resistance (Cu)

0.5 mm²37 mΩ/m

1.5 mm²12.1 mΩ/m

2.5 mm²7.4 mΩ/m

4 mm²4.6 mΩ/m

6 mm²3.1 mΩ/m

10 mm²1.8 mΩ/m

AWG 2254 mΩ/m

🔴 E24 Standard Resistors

1Ω 1.2 1.5 1.8

2.2 2.7 3.3 3.9

4.7 5.6 6.8 8.2

10 12 15 18

22 27 33 39

47 56 68 82

× 10ⁿ for any decade

⚡ Power Levels Guide

µW rangeSensor / RF

mW rangeLED / logic IC

1–10 WSmall appliance

100–1000 WDomestic load

1–100 kWIndustrial motor

> 1 MWPower station

Resistor max0.125–5 W

📚 Engineering Notes

Ohm's Law Assumes LinearityOhm's Law only applies to ohmic (linear) components — resistors, wires, and most passive elements at constant temperature. It does NOT apply to diodes, transistors, LEDs, or capacitors where the V-I relationship is nonlinear.

Power = Heat DissipationEvery resistor turns electrical power into heat. A 10Ω resistor carrying 1A dissipates P = I²R = 10 W. Always check the resistor's wattage rating — standard ¼W resistors will burn if P exceeds ~0.25W continuously.

Internal Resistance MattersReal batteries, power supplies, and wires all have internal resistance. Under heavy load, terminal voltage drops: V_terminal = V_open − I × R_internal. This is why battery voltage drops when you draw high current.

Short Circuit: R → 0When resistance approaches zero (short circuit), current I = V/R approaches infinity — limited only by internal resistance and wire resistance. This is why fuses and circuit breakers exist. Never short a supply without current limiting.

Voltage Divider PrincipleTwo resistors in series split voltage proportionally: V₁ = V_total × R₁/(R₁+R₂). Ohm's Law on each individual resistor confirms this. This is fundamental to sensor interfaces, ADC scaling, and biasing circuits.

Temperature CoefficientResistance increases with temperature for most conductors (positive temperature coefficient, PTC). Copper increases ~0.4% per °C. Thermistors use this effect deliberately. At high temperatures, your actual R will be higher than the cold value — leading to higher power dissipation.

What is Ohm's Law?

Ohm's Law defines the relationship between Voltage (V), Current (I), and Resistance (R) in an electrical circuit: V = I × R. Named after German physicist Georg Ohm (1827), it is the foundational equation of electronics. Combined with the power formula P = V × I, all 12 derived formulas for any unknown quantity can be solved from any two known values.

❓ Frequently Asked Questions

Ohm's Law (V = I × R) applies to all linear, resistive elements: fixed resistors, conductors, heating elements, and most passive components at constant temperature. It does not apply to diodes, transistors, or capacitors. It works for both DC and AC circuits, though AC circuits also involve impedance (Z) instead of pure resistance (R).

Use the formula R = V² / P. For example, a 100W light bulb on 230V has a resistance of 230² / 100 = 529Ω. Alternatively, first calculate current with I = P / V, then apply R = V / I. Both methods give the same result.

Voltage (V) is the electrical pressure or potential difference that drives current through a circuit — measured in volts. Current (I) is the flow rate of electric charge — measured in amperes. Resistance (R) is how much a component opposes that flow — measured in ohms. Think of voltage as water pressure, current as flow rate, and resistance as pipe diameter.

Use R = (V_supply − V_LED) / I_LED. For a 5V supply, red LED (2V forward voltage), at 20mA: R = (5 − 2) / 0.020 = 150Ω. Choose the nearest standard value — 150Ω is in the E24 series. Always check the LED's maximum current rating (typically 20–30mA for standard 5mm LEDs).

From I = V / R: at constant voltage, doubling the resistance halves the current. Resistance physically impedes electron flow — higher resistance means electrons collide more with atoms in the material, slowing their movement. This is the core principle used in current limiting, voltage dividers, and load resistors.

The basic power formula is P = V × I. By substituting Ohm's Law (V = I × R), you get two derived forms: P = I² × R (power from current and resistance) and P = V² / R (power from voltage and resistance). These 12 formulas together form the "Ohm's Law Wheel" shown in the diagram above.

When R = 0 (a short circuit), the equation I = V / R becomes undefined (division by zero), meaning theoretically infinite current. In practice, current is limited by the internal resistance of the source and connecting wires. This is extremely dangerous — short circuits cause overheating, fire, and explosions. Circuit breakers and fuses protect against this by interrupting current flow.

Yes, but with a modification. In AC circuits, resistance R is replaced by impedance Z (measured in ohms), which combines resistance and reactance: Z = √(R² + X²). The formula becomes V = I × Z. For purely resistive loads (heaters, incandescent bulbs), Z = R and standard Ohm's Law applies directly. For capacitors and inductors, the reactance (X) must be included.